1. Field of the Invention
The present invention relates to a sublimation type thermal transfer printer in which images are thermally transferred to a sheet of paper by pressing a thermal head onto the back of an ink sheet to sublime the ink, and to an ink sheet used with the printer.
2. Description of the Related Art
FIG. 1 is a side view of a sublimation type thermal transfer printer disclosed in Japanese Patent Application Laid Open No. 63-91281(1988). In the figure, the reference numeral 1 designates a thermal head that is rotated about support at one end. At the tip of the thermal head 1 is mounted an ink sheet separation roller 1a. A platen 2, a cylindrical rubber roller, is disposed at a position opposite to the heating elements of the thermal head 1. The platen 2 has a clamper 12 which clamps a printing paper 3 and presses the printing paper 3 onto the surface of the platen 2. The printing paper 3 is special for sublimation thermal transfer printing and coated with a plastic image-receiving layer for fixing the sublimation ink on the surface thereof. A pair of transport rollers 11 are disposed in the upstream of the platen 2, which delivers the printing paper 3 to the platen 2 or discharges the printing paper 3 outside the printer. The printing paper 3 is pressed and held onto the platen 2 by the clamper 12 and is transported to the transfer position or the discharge position as the platen 2 is rotated. An ink sheet 4 wound around a takeup reel 5a and a supply reel 5b runs between the thermal head 1 and the platen 2 in the direction shown by an arrow. In the vicinity of the tip of the thermal head 1 are disposed photosensors 9 for detecting a start mark on the ink sheet 4.
FIG. 2 is a side view of a printer disclosed in Japanese Patent Application Laid Open No. 1-290465(1989), which is similar to the one described above. The same or corresponding parts to those in FIG. 1 are designated by the same reference numerals, and explanation of such parts is omitted herein.
In the prior art of FIG. 2, the ink sheet 4 and the takeup and supply reels 5a and 5b are encased in a cassette 6, and the clamper 12 in FIG. 1 is replaced by a driving capstan roller 7 and a pinch roller 8 driven by the capstan roller 7, which transport the printing paper 3 to the transfer position or the discharge position. The printing paper 3 is pressed between the capstan roller 7 and the pinch roller 8 and discharged outside the printer. There are also provided a pair of photosensors 9 and 9 spaced apart in the crosswise direction of the ink sheet 4 in the vicinity of the tip of the thermal head 1. In FIG. 2, reference character A indicates the direction in which the printing paper 3 is transported during printing.
FIG. 3 is a plan view of the conventional ink sheet 4. Areas on a base film of the ink sheet 4 are sequentially coated with sublimation inks of yellow, magenta, and cyan in this order, in equal length shorter then the length of the transfer area of the printing paper 3. A start mark 4a coated with a black ink is interposed between the cyan and yellow areas on the ink sheet.
FIG. 4 is a plan view of another conventional ink sheet 4. The ink sheet 4 contains marks 4b, 4c and 4d respectively indicating the start of the yellow area, the magenta area and the cyan area, which are disposed at the beginning of the respective color area and each mark is coated with a black ink in different part from the other.
The operation of the first conventional printer using the ink sheet 4 of FIG. 3 will now be described. FIGS. 5, 6, and 7 are side views showing the printer in paper feeding, transfer printing, and paper discharging, respectively.
As shown in FIG. 5, the platen 2 is rotated to move the clamper 12 closer to the transport rollers 11 and 11 to position the clamper 12 at the paper feed position, then the clamper 12 is raised from the surface of the platen 2. Next, the transport rollers 11 and 11 are rotated in arrow directions as shown, to deliver the printing paper 3 to the clamper 12. When the leading edge of the printing paper 3 reaches the clamper 12, the clamper 12 is lowered to the surface of the platen 2 to firmly press and hold the leading edge of the printing paper 3 onto the platen 2. As the platen 2 is rotated, the printing paper 3 is wound around the surface of the platen 2 and is placed at the transfer position. At the same time, the takeup reel 5a reels up the ink sheet 4 until the photosensor 9 detects the start mark 4a on the ink sheet 4, thus readying the ink sheet 4 for printing.
Then, as shown in FIG. 6, the thermal head 1 is turned to press the ink sheet 4 onto the printing paper 3 held on the surface of the platen 2. When rotating the platen 2 in the arrow direction, the thermal head 1 is energized to sublime each color ink on the ink sheet 4 and transfer each color image to the printing paper 3. After the yellow ink is transferred, the magenta and cyan inks are transferred, in this order, to the same transfer area on the printing paper 3 which is moved back to the start position of the yellow ink transfer. When the clamper 12 passes beneath the thermal head 1, the thermal head 1 is temporarily turned upward to avoid contact with the clamper 12.
Since the length of each color area on the ink sheet 4 is shorter than the transfer area on the printing paper 3, each color ink is transferred a plurality of times for one transfer area by changing the transfer start position each time. The three color inks, yellow, magenta and cyan are thus successively transferred one on another to produce a full color print.
After the transfer process is completed, the thermal head 1 is turned upward, as shown in FIG. 7, and the platen 2 is rotated in the arrow direction as shown. The trailing edge of the printing paper 3 is then transported backward by the transport rollers 11 and 11 which are now rotating in the directions opposite to those in the paper feeding operation. Upon reaching the paper feed position, the clamper 12 releases the printing paper 3 which is then discharged from the printer only by the rotation of the transport rollers 11 and 11.
Next, the operation of the second conventional printer which uses the ink sheet 4 of FIG. 4 will be described below. FIGS. 2 and 8 show the printer in paper feeding and transfer printing, respectively, and FIG. 9 shows the printer in color change or paper discharge operations. In FIG. 9, the reference character B indicates the direction in which the printing paper 3 is moved backward when changing the ink color to be transferred.
With the thermal head 1 resting in standby position above the platen 2, as shown in FIG. 2, the printing paper 3 is delivered to the print start position as in the figure. The takeup reel 5a is rotated to reel up the ink sheet 4 until the mark 4b indicating the start of the yellow ink area on the ink sheet 4 is detected by the photosensors 9, thus readying the ink sheet 4 for printing.
Then, as shown in FIG. 8, the thermal head 1 presses the ink sheet 4 and the printing paper 3 onto the platen 2. The capstan roller 7 and the pinch roller 8 catch and press the printing paper 3 between and the pinch roller 8 follows the rotation of the capstan roller 7 thereby to transport the printing paper 3 in the arrow direction A. At the same time, the takeup reel 5a reels up and transports the ink sheet 4.
As the printing paper 3 is moved, prescribed electrical signals are applied to the thermal head 1 to selectively heat the heating elements in the thermal head 1. The heat is conducted to the ink sheet 4 so that the yellow ink is sublimed and fixed to the image receiving layer on the surface of the printing paper 3. When the yellow ink printing for one picture is completed, the thermal head 1 is moved apart from the platen 2 and is lifted upward as shown by dotted lines in FIG. 9.
The takeup reel 5a reels up to transport the ink sheet 4 until the mark 4c indicating the beginning of the magenta ink area on the ink sheet 4 is detected by the photosensors 9. The capstan roller 7 now is rotated in the opposite direction to the paper feed direction for printing, to move the printing paper 3 backward in the direction B. The capstan roller 7 is stopped to rotate when the printing paper 3 is positioned at the start position of printing as shown in FIG. 2.
Next, printing with magenta ink is performed in the same manner as the yellow ink printing. After that, as the start of the magenta ink printing, the printing paper 3 is moved backward to the print start position for printing with cyan ink. In this manner, the three color inks of yellow, magenta and cyan are sublimed and thermally transferred one on another to produce a full color image on the printing paper 3.
At the end of the transfer printing process, the takeup reel 5a is slightly rotated to move the ink sheet 4 by a small distance in order to take in the slack of the ink sheet 4 caused by the upward movement of the thermal head 1. Next, the capstan roller 7 is rotated in the same direction as in the transfer printing, to further transport the printing paper 3 in the direction A, thus discharging it outside the printer.
As described, the conventional sublimation type thermal transfer printer of the above construction requires the use of special printing paper consisting of a plastic image-receiving layer for fixing sublimation ink coated on a base film, and cannot use plain paper as in plain paper copiers. Therefore, the printing cost of color images is high.
On the other hand, a sublimation type thermal transfer printer capable of color printing on plain paper requires special control circuitry and control software for controlling the printer, which complicates the mechanism, increases the size of the printer, and requires a longer time to print out a color image.
Furthermore, with such printers, print quality is lower and less stable than the sublimation type thermal transfer printer that uses special treated paper precoated with an image receiving layer. Moreover, when using various kinds of printing paper, such as plain paper and special treated paper, of different thicknesses and surface friction coefficients, it is difficult, in such printers, to precisely control the transportation of every different kind of printing paper.